Hi Sevenox
I don't know which factors will affect the frequency of CABs in your world, but the number 1 generic response would be the number of changes, followed by the types of changes - what do they impact, what is their risk and urgency level etc.
Size of organisation is also a factor as is geographic distribution.
Also are the CAB virtual or are they held in a specific time and place(s)?
Did I mention politics?
To summarise, the answer is 42.
Did I say that? No, no, the answer is It Depends_________________DYbeach
ITIL V3 Release, Control & Validation,
ITIL V3 Operation SUpport & Analysis
PMI CAPM (R)

"In times of universal deceit, telling the truth will be a revolutionary act." George Orwell

CAB meetings should be held as frequently as is required to meet the needs of the business for whom the service is provided, within resource and budgetary constraints.

Alternatively:

CAB meetings should be held as frequently as the Change Manager deems them necessary.

If you want to know how often your organization should hold CAB meetings I can answer that in from a day's to a month's consultancy, depending on your budget, the size and complexity of your organization and the precision of answer you require._________________"Method goes far to prevent trouble in business: for it makes the task easy, hinders confusion, saves abundance of time, and instructs those that have business depending, both what to do and what to hope."
William Penn 1644-1718

Hey, if you have a multi level CAB structure, the frequency can be... well... quite frequent.

That's a relatively positive statement. Or is it the other way about?_________________"Method goes far to prevent trouble in business: for it makes the task easy, hinders confusion, saves abundance of time, and instructs those that have business depending, both what to do and what to hope."
William Penn 1644-1718

Mate, assess the need for a CAB based on the changes happening in the system....

say if most changes are very minor and with almost no impact - design a strong change model - put a standard change implementation process for these chaneg types and categorize them as standard change - so that you dont have to set a CAB meetings for these standard changes and yet you ensure that you have reviewed the changes and the implementer knows what is needed for the approval for these changes..

Other changes, will need a CAB - virtual, telephonic or Physical - which is to be decided based on many factors, like no. of changes, type of changes, size of change and the magnitude of its impact, your company history on impact of changes happened in the recent past, business demands - well these are factors that will help you assess - you as a chaneg manager - it is their duty to assess them and then record the timing and frequency of a CAB

Size of User Base
Size of Organization
Number of Ongoing Projects
Rate of business change
Business cycles (year end etc)
Urgency of changes
Quality of change planning
State of infrastructure components
Backlog of RFCs
Time set aside for each CAB
Number of of documented standard changes as % of overall work required

there are numerous influencing factors that influence the frequency of CABs these are just some of the obvious ones.